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How did aircraft communicate during early air travel?
During the initial era of air travel how do the pilots/Navigation officers would communicate with ground stations?
Do they use radio communication? Or Telegraph was used?

Given that telegraph requires wires, I doubt that they used that.

given that radio was available before planes I'd say radio

@ratchet freak: I'm pretty sure that the earliest planes did not have radio, including at least some WW1 fighters.

then they could have used light signals (it's still the radio backup today)

There was such a thing as wireless telegraph, but of course it's a form of radio that only carried Morse code instead of voice.

There's lots of information in this PDF (APPENDIX 1. Signals.) on visual signals such as light, pyrotechnics, and wing waggling http://www.icao.int/Meetings/anconf12/Document%20Archive/an02_cons%5B1%5D.pdf

Well, obviously they used pigeons.

Radio was not widely used until the 1930s, before that, mostly light signals were used. Early ground support aircraft in WW I could drop little notes which were pencilled by the observer, stuffed in a small capsule to which a streamer was fitted, and that was dropped as closely as possible to the troops which should get that message. This method was still used in WW II, when most aircraft had radio, because only specially equipped ground troops had compatible receivers.

The first regular flights by night were 1921 for the air mail connection between Omaha and Chicago, and electric flood lights were positioned along the flight path.
When Lufthansa opened the first night connection between the Reich and East Prussia in 1926, they posted burning oil drums along the way to mark where the pilot had to fly. Communication the other way (air to ground) was not possible on those flights.

The first air-to-ground radio communication used morse signals, and this is the origin for these strange three-letter names for things like local air pressure at the ground (QFE) or pressure corrected to sea level conditions (QNH). There was a whole lot more of those, including some to order coffee before landing, but they are no longer necessary. Read all about it here.

Order coffee before landing - that sounds mighty useful.

Radio telegraphy was used for artillery spotting in WW1.

@DJClayworth I'm sure I recall reading about some form of air to air visual communication protocol which allowed spotter to relay information from an aircraft without radio.  But I've no idea of the source.  Some form of Morse probably.

Flying boats in airline service as late as 1950 or so, as well as in military service during WWII, were equipped with signal lights like those commonly seen on ships in war movies, used through the navigation dome above the cockpit (where the navigator took sun or star sightings to fix location).  How effective they were from aircraft to aircraft is questionable, and they'd only have been visible from another aircraft at or above the same altitude in any case.

@ZeissIkon That was particularly popular on British airplanes and copied from naval traditions. Other nations used more modern means.

While radio was available (meaning it existed) it did not exist in a form usable by early aircraft.
If you are talking early-early travel, not commercial travel, communication was not necessary. Navigate to the destination, circle the (farmer's) field to check for livestock, then land. Communication with other aircraft was not necessary as there usually were no other aircraft.
As things progressed, morse code became practical. This was when cockpit crews required both hands to count.

A little more detail might be helpful.

Up until World War One, there was no radio communication between air and ground, or between planes. Communication from ground to air was done by means of light signals.
From 1915 onwards aircraft were used as artillery spotters which involved Morse code wireless signals one-way,  (from the aircraft to the ground). There was no communication from ground to air, and the air-to-ground communication was limited to simple codes. The only form of air-to-air communications was signals such as 'wing-waggling'.
Air-to-ground and air-to-air spoken radio communication became common between the wars, and was widespread in major air forces at the start of WW2. It was used for air-to-air communication and short range air to ground, for example controlling interceptor fighters within the home country. Morse code remained in use for longer ranges, such as a bomber over a foreign country communicating with home.

In World War 1 there were aircraft spark gap transmitters and the pilot could send messages in Morse code to receivers on the ground but he could not receive wireless (now called radio) signals. There was limited communication from ground to aircraft using the following three methods;
Ground Strips.
Lamp Signals.
Poppet Panels.
The pilot had to fly and watch for signals at the same time.
Ground strips were white sheets 3 metres by 300 mm (10 feet by 1 foot) or larger. They were laid out as patterns or letters with a letter for such messages as 'Your (wireless) signal is weak' or, 'Repeat last (wireless) message' or 'I am not receiving your signal'.
Lamp signals, used later in WW1, used 'Lucas' signalling lamps. The operator aimed the light beam at the pilot when he thought it would be visible to him and send the message in Morse Code.
The Poppet Panel was a canvas shutter arrangement fixed to the ground and was worked by pulling a cord which made a white patch visible from the air. When the cord was released the panel showed a black patch. By pulling and releasing the cord the operator could send Morse Code messages to the pilot who had to fly with one hand, look over the side and write down the letters with his other hand. It was a difficult task.

Source for this?

@SMSvonderTann  The source is three books written by WWI wireless operator, Bert Billings. He served in Gallipoli and Egypt then transferred to the Royal Flying Corps and trained pilots. The only copies of his books are in the Australian Army Signals Museum in Victoria and the War Memorial in Canberra. I have just submitted a 14,200 word article based on his books for publication. I am the former manager of the Signals Museum so have access to the books. Google the various communication systems that I have briefly described in my answer. You should find some additional information.

You might want to put that at the end of you post saying: "Source: " And the title of those books.

@SMSvonderTann. Thanks for your advice but I don't see other postings referenced with sources. If you are interested in the subject do a thorough search on Ground Strips, Lamp Signals, Poppet Panels, Lucas' signalling lamps, with additional terms such as WW1, aircraft, artillery spotting, Hubert (Bert) Billings etc and you should find other sources of the same information. The only two copies of the three books by Bert Billings, apart from copies that members of his family have, are in the Australian Army Signals Museum in Victoria and the War Memorial in Canberra, Australia. They are My Story

Military aircraft frequently used radiotelegraphy (Morse code) well into WWII, if not 'til the end. The binary nature of it (on/off constant amplitude signal*) gave greater effective range and more resistance to degradation by static than radiotelephony (voice). Most aircraft had voice radios for communication with nearby friendly aircraft and bases, but relied on Morse code at long ranges.
* shades of digital transmissions today!


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Will the landing gear work mechanically in case of total power loss?
In case of a total power failure in all the aircraft systems like engine failure and APU failure, would it be possible to use mechanical means (manually) to open the landing gear bay door and deploy the landing gear through mechanical means?
I know it’s possible to glide the flight if the engines failed. But, wondering how they land.

Not a real example, but this comes from the FAA, including some nice drawings around page 13-20:

The emergency extension system lowers the landing gear if  the main
power system fails. There are numerous ways in  which this is done
depending on the size and complexity of  the aircraft. Some aircraft
have an emergency release handle  in the flight deck that is connected
through a mechanical  linkage to the gear uplocks. When the handle is
operated, it  releases the uplocks and allows the gear to free-fall to
the  extended position under the force created by gravity acting  upon
the gear. Other aircraft use a non-mechanical back-up,  such as
pneumatic power, to unlatch the gear.
The popular small aircraft retraction system ..... uses a free-fall valve for emergency  gear extension. Activated from the flight deck,
when the  free-fall valve is opened, hydraulic fluid is allowed to
flow  from the gear-up side of the actuators to the gear-down side  of
the actuators, independent of the power pack. Pressure  holding the
gear up is relieved, and the gear extends due to  its weight. Air
moving past the gear aids in the extension  and helps push the gear
into the down-and-locked position.
Large and high performance aircraft
are equipped with  redundant hydraulic systems. This makes emergency
extension less common since a different source of hydraulic  power can
be selected if the gear does not function normally.
In some small aircraft, the design configuration makes  emergency
extension of the gear by gravity and air loads  alone impossible or
impractical. Force of some kind must  therefore be applied. Manual
extension systems, wherein  the pilot mechanically cranks the gear
into position, are  common. Consult the aircraft maintenance manual
for all  emergency landing gear extension system descriptions of
operation, performance standards, and emergency extension  tests as
required.

in other words "it depends on the aircraft". Of course you can always attempt a belly landing, and then there's aircraft with fixed landing gear.

There's also the possibility that the ram air turbine will generate enough power to lower the gear without resorting to mechanical means.

@DavidRicherby i'm going to expand on the answer a little later with examples if i got time.

Gimli Glider used gravity drop according to Wikipedia

LOT B763 at Warsaw on Nov 1st 2011 landed gear up because a popped circuit breaker prevented alternate extension. Proves that in large planes some electricity is needed even for the alternate extension. But then electricity is needed to keep the hydraulics pressurized too, that's why there is the RAT.

The rulebook for transport category aircraft says
14 CFR 25.729

c) Emergency operation. There must be an emergency means for extending the landing gear in the event of—
(1) Any reasonably probable failure in the normal retraction system; or
(2) The failure of any single source of hydraulic, electric, or equivalent energy supply.

I'm not sure total power failure counts as either, though.

The F-16 has an "emergency blow-down" reservoir containing approximately 3,000 psi gaseous nitrogen. Hitting a switch will release this pressurized nitrogen into the hydraulic system dealing with the landing gear. Ergo, hitting that switch will extend the gear, even with the engine off.
So yeah, the landing gear will still work in the event of a complete power failure. It has backup systems.
In the older A-4 Skyhawk, the landing gear retracted up and forward.  So, in the event of a hydraulic failure, if you could get the gear doors open, the airflow past the aircraft in flight would "drag" the gear down and aft and lock it into position.  I think that's a pretty slick design.

The Twin Comanche I used for my AMEL rating had an interesting mechanism.  First you had to disconnect the electric motor, then you  stuck this handle into a hole and crank it as far as you can.  Then you shift it to a second hole (that becomes usable after the first one is used) and crank on it again until it stops.
Not something you get to try until you need it though.  Once you pull the disconnect it requires an A&P to restore the system to normal operation.

Is that the one that takes like 30 or 40 cranks to get the gear down?

Almost all aircraft have some sort of backup system for getting the gear down.  On the planes I have owned they were:

Manual hydraulic pumps to pressurize the system with a car jack type handle.  You sort of pumped the gear down.  Obviously does not work too well if the problem with gear system is a no fluid (blown line or seal).
Mechanical crank (21 turns to get down, 2 more to lock) that drove the gears if the electric motor died.  Good workout.

All aircraft have some kind of backup system.
As mentioned before- Skyhawks Landing gear gets sucked down.
most of the other planes- just fall down thanks to gravity.
A lot of the airplanes (including military and some civilian)- have a handle to open the bays mechanically (like in the 737) and the gear falls down and locks automatically with a mechanism that is independent of all hydraulic or electricity.
As for the big jets- they have an amazing amount of backups.
And still- if in a 787 all your computers catch fire for some reason- probably you wouldn't be able to extend your gear. But then- even flying the plane would be a challenge, as the aircraft needs voltage for everything.
In this case, there is another backup for flying the plane using a special connection of direct wires from the stick to the flight control surfaces, powered by backup generators.

It all depends on the airplane in question.
Typically for small GA airplanes using an electro-hydraulic power pack for actuating the gear, they’re held retracted by hydraulic pressure.  If the system fails, the pressure goes to zero and the gear just drops down due to gravity.  I have time in the DA-42 and DA-62 twins and those airplanes use an alternate gear extension handle - basically an emergency valve which releases hydraulic pressure and allows the gear to drop down on their own.
Electrically powered landing gear can be typically lowered by means of a hand crank in the cockpit.  This is prominent on Mooneys and light Cessna twins.
Larger airplanes and jets may make use of emergency reservoirs of compressed nitrogen called blowdown bottles to provide emergency hydraulic pressure and extend the gear.  Typically the landing gear retraction jacks can be isolated from the main hydraulic system via a cockpit switch to prevent total loss of hydraulic fluid in the event of a leak.  It also removes the additional workload from emergency systems eg APU, RAT, etc.  for more critical flight control functions.
Transport category airplanes often have multiple layers of redundancy ie multiple hydraulic systems and power sources to get the plane down safe.

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Can an intercepting fighter jet force a small propeller aircraft down without completely destroying it?
A rogue aircraft may fly into a no fly zone, and be intercepted by standard military procedure. That involves increasingly intense attempts to establish communication. When all these fail, it may be required to force the aircraft to go down. The naive solution is to just shoot it down, which may be the right thing to do.
I wonder if it is possible for a F-16 fighter to force a small one propeller airplane to do an emergency landing? Either by making it try to get to the nearest airfield, or alternatively making it try to get to the nearest cornfield - or anything somewhat flat.
For example by disabling its propeller. By either breaking the propeller by  shooting at it, or shooting holes into the engine?
Would that work for a small aircraft?
Would it work more generally, like for commercial airliners?

I think it would be very hard to target something like the propeller or engine in something like a 172. Would be easier for a small twin. Usually they put a couple "shots across the bow" or "thumping". They also visually identify the condition of the pilot (if they are slumped over or dead), then they will decide to end things by force.

Shooting the engine would not be possible without a 100% chance (rounded) of killing the pilot. You'd have a better chance trying to shoot the fuel cap off.

If you're shooting at it you are trying to kill it, period.

@JohnK Theoretically, it is possible that I have no interest in killing the pilot, but need to make it not fly into some area a mile ahead. Not sure that is practically possible to do, but the intention can exist in practical cases.

Well as a general rule of firearms, if you are shooting at something, you intend to kill it. Nobody shoots projectiles at anything intending to disable and no more, because the 1st round out could kill it, or the 50th. And pretty much anything I might do to physically affect the target airplane, like flying in front of it to force it to fly though my wake, also creates the probability that I may force it to crash, so if I take any action like that, I've decided in advance to kill the target if necessary, and if it's only forced down, that's a bonus.  Bottom line is it's all potentially lethal.

This question reminds me of complaints by people who have never used firearms that police shoot to kill instead of "shooting the gun out of the perpetrator's hand".

It also makes me think of Mathias Rust.

@RonJohn Yet in e.g. Finland, it is exceedingly rare that police shoot to kill (7 dead out of 122 shot in past 10 years). It's not that impossible to resolve such situations without killing.

@jpa "7 dead out of 122 shot in past 10 years" does not mean that Finnish police shoot the guns out of people's hands.

@jpa A very important factor for whether a shot kills or not depends on the properties of the projectile. Speed is extremely important. A policeman usually uses his handgun, which shoots something like 9 mm projectiles relatively slow. An assault rifle shoots projectiles of similar size, but with much higher velocity. To compare it, roughly, the handgun shoots holes into organs. So after a shot in the liver, a surgeon can open the patient and try to get the surrounding  parts provided with  sufficient blood etc. When it is shot with an assault rifle, the surgeon may not find a liver at all.

@jpa The assault rifle projectile creates a large temporary cavity, and strong shock-waves. That can turn the liver into liver colored goo, and directly after the projectile passed, there was a vacuum cavity at that location.

So, back to handguns: They make holes in people. And people can live a while with holes in many places. A person dies from shot wounds mostly because of internal bleeding. And when police shoots somebody, medical help is quickly available before that happens. Also, police usually does not aim for the head, which avoids holes in brains.

An interceptor can gain cooperation from an intercepted aircraft by means of a show of force ie forming up on the target’s 6 and 9 o clock position and attempting to contact them on the emergency radio channel.  If the target remains unresponsive or refuses to comply, a fighter will often cut in front of its flight path in afterburner, causing the target aircraft to be rocked and buffeted in the jet wash.  This is usually enough to get a response from the target aircraft and compliance with the lead fighter’s or ATC’s instruction.
Any kind of intentional damage to the intercepted aircraft would not be done unless deadly force were authorized against the target.  In which case full force ie employment of weapons against the target aircraft to shoot it down would be used.
The video below is a good report on interception procedures against a civilian aircraft.  It was a joint exercise between the civil air patrol and an F-16 ANG unit.

The interesting part starts at 2:15.

Good point, any kind of physical interaction with the intercepted  aircraft requires authorization for deadly force. That's because any physical interaction has a high probability to kill all people on board.

But what to do when I only need to prevent the plane from flying the next three miles. I do not need to kill them, if I can bring it down by letting it emergency land or just crash - with some chance to survive. For example, I can see the pilot, and he seems to be passed out. Possibly a young child tries to control the aircraft, and is just hopelessly over-strained. I have no reason to actually kill them, but also, I can not be sure it's not a disguised attack.

@VolkerSiegel Your scenario makes me think of this incident from yesterday: Trainee pilot lands plane safely after instructor passes out. Of course, the circumstances were different as the student communicated.

@VolkerSiegel That scenario is going to get twisted fast.  Really soon it will start to matter why there is a no fly zone, and what situational awareness the jet pilot has.  What works against a passed out pilot may not work against a pilot who is actively trying to deceive you into thinking they are passed out.  What exact aircrafts are involved?  Also, can they afford to permit an individual aircraft to violate the no-fly zone and just let it safely fly by?  The number of variables that matters in that moment are astounding.  Such a pilot has to consider all of them.  Poor soul.

@CortAmmon It's a constructed example for a situation where I need to get the airplane out of the air, but have no reason to kill the pilot. Basically as argument that the situation can exist. It is an example situation where 1. the airplane needs to get out of the air in a finite time or space, but not immediately. 2. It is acceptable to kill the pilot, so using a AIM-9 Sidewinder air-to-air missile is a potential solution for the problem. There is no reason to kill the pilot, so it would be advantageous to find a solution that has some chance to not kill the person, even if it is small.

What would happen if the intruding aircraft were armed? I cannot imagine an F-16 pilot deliberately putting himself in front of an MiG flying over its territory.

If it was determined that the unknown contact was a hostile military threat, the rules of engagement change quite a bit. That’s a totally different animal than an errant civilian pilot in a Cessna who didn’t bother to read the NOTAMs before flying.

@dotancohen there are other ways to get the attention of military aircraft. eg Locking a military aircraft up with fire-control radar and setting all its threat warning systems screaming is an extremely vocal way to send a "HEY YOU!!!!!" message.

This explains the incredibly interesting context - but it does not answer the question. The assumption is that all communication attempts have failed. That the authorization for killing is required is true, but the fighter pilot is not required to kill to complete the mission. If a sidewinder air-to-air missile of him would leave the pilot and a parachute on his back as one piece, the fighter pilot would not be required to kill the pilot after he touches the ground.

@Volker Siegel That’s not how use of deadly force works.   You’re not actively trying to “kill the pilot“, but you are employing deadly force to stop the threat. Deadly force by definition is force which is likely to result in serious injury or death.  There is no known way to bring the target airplane down, without the cooperation of the PIC, without the use of deadly force being used.  Any kind of damage one would intentionally inflict on the target aircraft is likely to kill or seriously injure the pilot.

The jet fighters could harass the small plane, to the point of making it very clear: You will not get much farther on this path!
Note what happens when the F-14s fly under the Zeroes here:

The Zeroes aren't waggling their wings.  This is an upset - they are being flipped over by the wake vortex from the Tomcats. The competent pilots are then using aileron to arrest the upset.
The same effect is examined in great detail here. In both cases the Zero and DR400 are able to arrest the upset at 90 degrees, but tragically the DR400 is much too low. Its wake turbulence was from an An-2 biplane (1/4 the weight of an F-16 and 1/13 the top speed) which had taken off 40 seconds earlier. TURN YOUR SOUND DOWN:

Anyway, 2 or 3 fighter jets could hit this guy with that kind of upset every few seconds, and from different angles, diving for instance to put him in a spin.  He'd be at 100% workload simply recovering from upsets - he'd be so busy trying to put the horizon line back where it belongs, that he'd have no chance of even knowing which compass heading he was flying!   As soon as he's pointed away, the fighters leave him alone, and if he turns back to his heading, suddenly green and blue are sideways again!
Inducing these upsets is inherently rather dangerous, so the fighter pilots wouldn't do it unless deadly force was authorized.
Further, the fighter pilots would have a real job matching their harassment to his ability to recover.  They could very easily create an upset which was totally recoverable, just not by that pilot.  The longer they have to do this, the better; ideally they'd start by throwing "softballs" at him, and if he handles those easily, turn up the heat until they find his skill limits.
What if the fighter is unarmed?
Obviously, the fighter jocks could induce these upsets with an Antonov-2. So it's no trouble in an actual fighter.  They have even more options, even without weapons.
The fighter could make a high subsonic or supersonic pass from behind very close, inducing such extreme turbulence as to snap the wings off.
The fighter could intentionally collide with the light plane's control surfaces; the 30,000 pound fighter's wing leading edges are far less likely to take critical damage than the 2000 pound small plane's trailing edges.
Finally, the fighter has the "blank" ammo they carry for weight and balance reasons.
Fighter jets don't shoot bullets, they shoot shells.  Shells have a fuze and bursting charge, which explodes the shell into a cloud of shrapnel, doing devastating damage.  This is necessary due to the speed of modern aircraft.  Fighter jet "blank" ammo has propellant, but no fuze or bursting charge, it's just a slug -- so yeah, it's a bullet.  It passes through the airplane's structure, leaving 2 or 4 holes and wrecking anything in its path.  Usually nothing, but if it gets lucky, it hits a control wire, spar just right, human or the engine.  At 50 rounds per second against a 150 mph foe, they don't need to get all that lucky.

A number of related questions has been asked and answered, addressing specific elements of interception:

What's the point of intercepting an aircraft?
How does a fighter “thump” an intercepted aircraft?
What is the lowest-speed bogey a jet fighter can intercept/escort?

A fighter wouldn't shoot at a private or commercial aircraft without intent to (potentially) kill. Any projectile damage to an aircraft is potentially fatal, due to how it can interact with the pilot's control inputs, even if one had a precise weapon.
Modern fighters guns generally use HE/HEI rounds, which have an explosive filling and produce fragmentation from the shell. There is no way to ensure a hit on just one and only one piece of a plane with such a round. Any fragment hitting the pilot can be lethal, and the pilot's dead body can finish the job of crashing the plane.

You are not going to be able to specifically target something as small as a C-172's engine or propeller with an F-16's gun. Even an actual sniper wouldn't be able to hit a moving target that small from a moving platform. If the F-16 even tries it, then they almost certainly will wind up shooting the plane down.
A similar situation exists with trying to force them down with your jetwash. Sure, it can cause them to lose altitude, but only for a short period of time. The other airplane could easily gain back the lost altitude and then some while you're repositioning for the next attempt. Even if there was some way to keep the other airplane in your jetwash for a long time, that would just result in them smashing into the ground rather than landing.
So, no, there's no way to physically force another airplane down without killing it. You can threaten them over the radio, fire warning shots, etc., but if they ignore the warnings, your only choice is whether or not to follow through with the threat.
For larger commercial jets, the situation is slightly better, but not much. An F-16 at point-blank range could conceivably hit just an engine with their gun. But there's still a huge risk that stray bullets or debris from the engine itself could tear up the wing, disable flight control surfaces, or pierce hydraulic lines. So, still pretty likely to result in a crash.

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text:
What is the fluid released from the tail of this clipped Turkish airliner?
A Turkish A321 airliner was hit by an Asiana A330 on 13 May 2018 at Istanbul Airport. It seems a large amount of fluid was released from the damaged section. What is the fluid? There is no APU up there so only hydraulic fluid, am I right?

Some news: https://onemileatatime.boardingarea.com/2018/05/13/turkish-plane-tail-istanbul/

That's the plane peeing itself after getting hit by the other plane.

It is hydraulic fluid. This works on 3000psi so that would explain the geyser.
I am not familiar with the A320 family but on most aircraft hydraulic system there are usually isolation valves to ensure that part of the system remains serviceable if there is a sudden loss of pressure/fluid in one part of the system.

I think you are talking about hydraulic fuses and they are normally on the landing gear to retain pressure for the other brakes if a line blows.  There wouldn't be anything like that on lines going to the rudder system.

is that really 20 MPa, i.e., 200 atm?

3000psi for most aircraft. The A380 works with 5000psi.


text:
Who logs the flight hours for airline pilots?
Airline pilots' flight hours vary a lot depending on the airline, type of operation etc. Who actually does the job of logging their flight hours? Do the pilots do that themselves after each flight or there is a system that does it for them?

At my outfit, both.
Pilots log their own hours in the logbooks and keep a rough mental track of hours flown to ensure compliance with Flight Time Limitation rules.
The airline will also log pilot hours to stay in accordance with FTL and compare rosters to actual hours flown. This also serves to create a legal record of who was flying the aircraft should that be required at a later date.
Many - including myself - use an electronic logbook that pulls the data from the airline, so no writing needs to be done.

I also use an electronic log book that pulls data directly from the airline. I suspect it is pretty common these days. My airline also maintains software that is used by by pilots to "check in" for work and will give a warning if the pilot is approaching recently limits.  ( eg. 3 TO and Landings in 90 days)

The pilot is responsible for logging (and tracking) his own time, and is personally responsible for managing his FTL. Most airlines cross-match this with OOOI data from ACARS regularly. I'm not sure how they track pilot-in-command time electronically though, as this is done verbally on the flight deck during flight. Does anyone know?

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text:
Were civilian aircraft allowed to overfly the Soviet Union?
The great-circle route from Western Europe to, say, Japan involves putting in a lot of miles above Russia, for example like this (London Heathrow–Tokyo Narita; map from gcmap.com).

Would routes like that have been possible during the days of the Soviet Union or would civilian planes have had to take a more circuitous route, flying south of the USSR and maybe even avoiding the Warsaw Pact countries, too?
I'm aware that at least some of Soviet airspace was prohibited which, for example, led to the Soviet Air Force shooting down Korean Air Lines flight 007. Presumably, the route shown in the map would be prohibited, since it goes over the Kola Peninsula, where the Northern Fleet is based and Novaya Zemlya, a major nuclear weapons test site.

Passes pretty close to another shootdown.

@Pondlife *sigh* There is no need to edit the title so it's a literal question with a literal question mark. There was nothing at all wrong with the original title. (There's nothing wrong with the new title, either, but the edit was unnecessary.)

@DavidRicherby I don't want to rehash the meta discussion here. It's a subjective thing and I don't claim to be right, but if you feel strongly about it then you can simply roll back.

BA used to operate LON-ANC-TYO right up into the 1980s. There was also a tag-on to OSA. Later they got permission to stop and refuel in Moscow instead.

During the Cold War, Soviet airspace was very restricted to other countries. Flights from the US often had to follow polar routes to Asia and Europe that avoided entering Soviet airspace. The end of the Cold War allowed new routes to be used.

Source
Flights to the USSR were allowed on a limited basis, often in conjunction with service by Aeroflot. Air Canada had the first service (via Copenhagen) from North America in 1966. Some flights were allowed to cross the USSR (JAL started in 1985) but would often stop on Moscow. Otherwise, Anchorage was a typical stop on such a route. International relations may have improved since the Soviet era, but even today not every airline gets permission to overfly Russia.
The strict policies are the reason that flights such as Korean Air Lines flight 007 were shot down when deviating from these routes, and the flight of Mathias Rust to Moscow was a big deal.

Flights were permitted, IF they had prior clearance. KAL007 had not requested clearance as they had no intent of overflying the USSR and did so accidentally, Rust of course was a romantic idiot who had no clear idea of the implications of his action.

@jwenting But in what circumstances was clearance available? Presumably not routinely or all those flights wouldn't have gone miles out of their way to Anchorage (for example, LHR-ANC-NRT adds 2,000 miles, or 33%).

The "not every airline... " link is broken.


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Is an EASA STC valid if it mentions issue 1 of a Master Drawing List that was later revised?
EASA STC certificate mentions the original issue of the Master Drawing List of the STC as "Associated Technical Documentation". But the STC provider delivered a revised version of the MDL. Is the certificate which refers to the original issue still valid or shall I request a revised certificate for the STC? Can anyone advise me on this?
Thank you in advance.

This sounds like a case where I would contact the STC holder and ask them what's up.
In the USA I'm used to seeing STCs with the magic words or later approved revisions (which means that the manufacturer can revise the document and get it re-approved without needing a new STC), but if a specific revision is called out and those magic words aren't present then later revisions are no longer "approved data" (or whatever the EASA equivalent term is) and technically can't be used.
In all likelihood the two revisions are materially identical and this is just a "paperwork problem", but you're better off being sure (and either getting a STC with the "later approved revisions" wording or getting the original issue of the drawing list so you have it in hand and comply with the STC you have) rather than having your local regulatory authority decide your installation is illegal because you didn't use approved data.

If there is a changelog on the document in question your mechanic could theoretically make the determination themselves - Added drawing #123 has no material effect if your installation doesn't reference that drawing. At that point it depends on how picky the folks approving your installation are though - again, they could say since it's a later revision it's not the approved revision.


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When does the use of afterburners save fuel?
An afterburner uses a lot more fuel for a little more thrust (rule of thumb: 50% more thrust for 5 times the fuel flow with full dry power). Is a flight maneuver possible where using the afterburner will result in less fuel use than flying the same maneuver with dry thrust?
To be precise: I am looking for a situation where the aircraft transits from state A to a desired state B. The transition is possible with both dry and wet thrust. Please describe a pair of A and B where less fuel is used in the transition when the afterburner is lit!

I'm speculating that if such a pair of states does exist it might be straddling a Mach boundary. e.g. A subsonic & B trans-sonic. Or A trans-sonic & B supersonic etc? Again, I've no reasoning only an intuition.

@curious_cat: Absolutely right!

Didn't Concorde use burners during trans-sonic flight for that reason (i.e. to spend less time in the high-drag trans-sonic regime?)

So, you're testing us, Professer @PeterKämpf? Sheesh, nobody told me there would be a quiz today...

@PeterKämpf: My naive reasoning was: To get from any point A to B in parameter space should usually be a state function i.e. path independent. i.e. You can give many tiny increments or one large increment. Except when a dissipative force exists. Now you no longer have a state function. Since drag, a dissipative force, is indeed a strong function of speed it will help to not linger in the high drag region i.e. transsonic with wave drag.. And that would give a high thrust afterburner the advantage needed over a dry engine. Again, this is just an hand-waving argument. I could be totally wrong.

@FreeMan: No, this question was my reaction to the answers on TBBT's afterburner question. I wanted to counter the "afterburner = higher fuel consumption" mindset.

@curious_cat: If you put your reasoning into a proper answer, I'll accept it.

@PeterKämpf Done! I'm trying to put this mathematically now. Sometimes hand waving arguments can be dangerously seductive. Yet ultimately wrong. Rigor gives confidence.

@PeterKämpf I added some plots to my reasoning. Would love your comments.

@curious_cat: Looks good! Maybe you exaggerate the transsonic drag peak and underestimate the drag increase in supersonic flight, but qualitatively all is correct.

@PeterKämpf Thanks! What are good functional forms to compute drag in the transsonic region & supersonic region.

180 turn with return to the same point and speed. Aircraft doing Cobra maneuver may be more efficient than going a long circle.

If such a pair of states does exist it ought to be straddling a Mach boundary (I think)

e.g. A subsonic & B trans-sonic. Or A trans-sonic & B supersonic etc.
Reasoning: To get from any point A to B in parameter space should usually be a state function i.e. path independent.
i.e. You can give many tiny increments or one large increment.
In our context a parameter space means mostly a {velocity(v), altitude(h)} pair.
To go from A{v1,h1} to B{v2,h2} you need a certain thrust profile. The required thrust profile is not unique.
Now the above path independance statement is true in general, except when a dissipative force exists. Now you no longer have a state function.
Since drag, a dissipative force, is indeed a strong function of speed it will help to not linger in the high drag region i.e. transsonic region with wave drag. In this zone the drag coefficient increases rapidly and you will have a total drag much higher than at both sub-sonic and super-sonic speeds.
And that would give a high thrust afterburner the advantage needed over a dry engine: On afterburner you can zoom through the transsonic region  rapidly than trudge through it incrementally on dry thrust. Thereby saving on drag losses. You are still consuming more fuel per unit time but the time spent is reduced so much that it makes the higher specific consumption worthwhile.
Caveat: This is just an qualitative argument. A more mathematical argument is needed to add rigor to the analysis.
Edit: Maybe these schematic plots help clarify my argument.

This seems to be exactly how Concorde used afterburners, so it apparently does make sense in practice.

I realize this is very old, and not quite the exact same issue, but to my remembrance the X1 also used AB to go trans-sonic because the aerodynamics were not well understood at the time and there were severe issues with loss of control authority right around mach point. The solution found was to move above mach as quickly as possible.

Virtually never in most aircraft. Afterburners usually increase fuel consumption by a factor of between five and ten from maximum non-afterburning throttle ("full military power"), while the speed increase is often less than double.
There was one singular aircraft designed to use afterburners efficiently, the SR-71. Essentially it was designed with a "bleed bypass"; at speeds beyond about Mach 1.5, air in the first compressor stage was diverted behind the combustion chamber using a series of ducts, where the afterburner would then have cleaner air to burn. This drastically improved the efficiency of the engine's afterburner, making it effectively a ramjet design. I can't say whether its range was improved by the use of afterburners, but the pre-bypass speed was limited to around Mach 1.8 while the afterburning speed was greater than Mach 3.3, almost a 100% speed increase and unmatched by any air-breathing craft that has flown since.


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What is the cart and cable connected to this plane?
During boarding I noticed many times that planes are "connected" with some orange cable.

It's curious to know what is it. Does anybody knows?

The "Vote to Close" presumes that the answer to the other question is in fact the correct answer to this question, and while that might be the case, it's not clear to me that it necessarily is.  The cart in this picture looks entirely different than the cart in the "duplicated" question, and the cable is much, much longer in this picture. Voting to leave open, at least until something more definitive than voting-to-close shows that the answers are, in fact, the same.

@RalphJ technically it can be answered each time, as there are many cables which get connected to an aircraft during ground ops, and the photo in thus question depicts a different aircraft to the one in the linked question... they are both ground power, just in this case the cable isnt connected to a power source, and are bundled on some steps.

On further investigation, thats an A320 rather than a 757, and the A320 Airport and Maintenance Planning document shows that that hookup is the ground power port (page 206).
The stairs are for passing documentation up to the cockpit side windows, which open.


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What is the difference between P208 and P209 base runway materials?
Runways are made with P208 base or P209 base does anyone know why there are labeled differently from regular crushed aggregate base? And what the different uses are for?

From the FAA Airports Construction Standards Part 3 ('flexible base courses');
Just a side note that these are just little introductions to the materials. Download the word document from the linked page and just read that if you want.

P208: This item shall consist of a base course composed of course aggregate bonded with fine aggregate base. It shall be constructed on a prepared subgrade or subbase course per these specifications and shall conform to the dimensions and typical cross-section shown on the plans.
  The aggregate base material shall consist of both fine and coarse aggregate.  Material shall be clean, sound, durable particles and fragments of stone or gravel, crushed stone, [   crushed slag,   ] or crushed gravel mixed or blended with sand, screenings, or other similar materials produced from approved sources. The aggregate shall be free from lumps of clay, organic matter, and other objectionable materials or coatings.

And for P-209

This item consists of a base course composed of crushed aggregate base constructed on a prepared course in accordance with these specifications and in conformity to the dimensions and typical cross-sections shown on the plans.
  Crushed aggregate shall consist of clean, sound, durable particles of crushed stone, crushed gravel, [   or crushed slag   ] and shall be free from coatings of clay, silt, organic material, or other objectionable materials. Aggregates shall contain no clay lumps or balls. Fine aggregate passing the No. 4 (4.75 mm) sieve shall consist of fines from the coarse aggregate crushing operation. If necessary, fine aggregate may be added to produce the correct gradation. The fine aggregate shall be produced by crushing stone, gravel, [   or slag   ] that meet the coarse aggregate requirements for wear and soundness.

You can download the Word file for more info on this, if you want to.

Either that's some quick sleuthing or you know your runways!

I know that much of it, but is there different base for different runway weight requirements? That is the only thing i can think of it, that the different gradations hold up different to different weight planes, and i just having found a clear answer on that.

@user22258 I the question 'can the plane on this kind of runway' is not only based on the P-2xx base that is used, but also the layers above that base

But I don't know anything about weight requirements for certain materials, with even the possibility that they are all the same. Maybe mentioned in that Word doc?


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Why do uncontained engine failures still occur?
One of the absolute requirements of an aircraft turbine engine (usually some sort of turbofan or turboprop) installation is that, in the event of a destructive failure of the engine, the engine cowling must be able to contain any and all fragments released in the process.  In layman’s terms: engine blow up, engine parts stay in cowling.  The cowling’s ability to contain an engine disintegration must be demonstrated in testing.  All of this has been the case, without exception, for decades.
And, yet, uncontained engine failures continue to occur.  As recently as October 2016, a 767’s engine exploded during takeoff.  (Yes, I know about the one with the 737 earlier this year, but that one’s still under investigation, and, as such, off-topic until the NTSB releases their final report.)
Why is this?  It can’t be for lack of testing capability, as the engine manufacturers can and do – indeed, are required by law to – blow up engines in their test stands to verify their inability to escape their cowlings, and causing an engine failure for such a purpose is ridiculously easy: wrap some detcord around a fan or turbine blade (to test against the engine throwing a blade), tie it to a fan or turbine disk (to test against one of the rotors seeing fit to come apart in flight), or wrap it around the engine shaft (to test against a shaft separation and consequent turbine overspeed and disintegration, LOT 007-style), run the engine up to full blast, and push the button.  So why do engine cowlings still sometimes fail to contain rapid unplanned engine disassemblies?

Because the universe isn't a perfect place.

I am almost sure I've already answered almost identical question around here. The related question search, unfortunately, does not work well, so lets see if I can find it…

@JanHudec This one?

There are no "absolute" requirements.

@Pondlife, yes, looks like it. So it was not that identical after all, and I only commented—but it did discuss this topic.

One of the absolute requirements of an aircraft turbine engine (usually some sort of turbofan or turboprop) installation is that, in the event of a destructive failure of the engine, the engine cowling must be able to contain any and all fragments released in the process.

No, there is not. The requirement is that the engine cowling must be able to contain fragments released in case of a single blade failure.
If just a blade fails, it will often break more downstream, but as long as it is just blades breaking loose, the casing should be able to stop them and generally does.
However if the disk that holds the blades itself breaks, the energy is much higher and the cowling can't stop that. It is not really possible to make it strong enough to contain this as it would be too heavy for flight, so it is not a requirement.
All¹ the recent cases of uncontained engine failures were that the whole disk broke and left the engine in several large pieces.

¹ The Southwest flight 1380, B737 near Philadelphia on Apr 17th 2018 is a kind of exception. It was only a blade failure, but it was also initially contained. The blade that failed was actually stopped by the cowling. However then a secondary failure of the inlet cowling itself, well ahead of the fan, occurred and that was what caused the further damage and injury.

Also notable is EASA CS-E 510 (a)(3) saying you show by analysis less than one high energy uncontained failure per 10^7 flight hours.

It's been pointed out that a single blade may be contained, but having a whole stage fail is an extremely high energy event. The high speed spool of a turbine engine is spinning at 10's of thousands of RPM. The energy in that system is too high to contain economically.
But you can't just have engines exploding and do nothing about it. As with any safety issue, it comes down to a negotiation between the manufacturer and the regulators. If the failure can't be contained, you have to mitigate the risk some other way.
Manufacturers look at how a failure is likely to occur and do their best to protect critical systems, either by routing them elsewhere or by shielding local areas. The FAA has published AC 20-128 to address this. It's particularly important that the other engine of a twin-engine aircraft is protected, as well as the hydraulic systems, and critical structure.
Uncontained failures are still taken pretty seriously by investigators, and they work to find answers so that future occurrences might be prevented.

+1 for "economically.  But how about just a heavier strip at the top to help prevent the blades from shotgunning into the wing?

@MontyThreeCard the energy you need to contain in a single released blade is about the same as a medium sized car, with passengers in every seat, travelling at about 80mph and crashing head on into something rigid like the concrete support of a bridge. For multiple blade failures, if the complete fan disintegrated, that means not just one "car" to stop, but about 25 crashing at the same time. Think about how big your "heavier strip" would need to be to deal with that situation, and whether you think it is economically possible to add such a thing.

Just to add to this, the wing and fuselage are also designed to be able to absorb damage (again as far as reasonably possible).  This is one of the advantages of mounting engines under the wing, instead of above the wing or around the tail.  In case of explosive failure of the engine, shrapnel has to get through the wing and fuselage before it'll hit occupants, or most critical systems.  It also keeps the vertical stabiliser out of the way of shrapnel.  As you say, the point isn't perfection, it's just to stack the deck really heavily in your favour.

Ok, but using the same logic, ANY reduction of the energy of the blade fragments would greatly reduce the effects on the wing, stabilizer (and cabin!).  No, I don't think a single strip would prevent all damage, but lessening the force might reduce the effects.  https://en.wikipedia.org/wiki/Qantas_Flight_32

To make it absolutely impossible would mean creating an engine casing so thick and heavy it'd make it pointless to have the engine in the first place as it would barely if at all be able to lift the engine casing, let alone the entire aircraft.
So compromises have to be made, and that means designing things where the chances of a blade detaching at high speed are minimised as much as possible unless other catastrophic events are also happening that would bring the aircraft down anyway.
That's always the case with engineering. The perfect solution for one set of requirements tends to lead to something that's impractical to say the least in reality, therefore you have to trade off something for something else and come up with a working solution that gets the job done within the parameters described and is the best possible solution everywhere else within budget (be it energy, cost, size, risk, or usually a combination of those).
That's why modern nuclear power stations are so large and have massively thick concrete domes over the reactors. That's not for any scenario that's likely to happen in real life, it's for the extremely remote chance that a large asteroid falls onto the dome, or someone flies a large aircraft into it at high speed.
For those things, weight and to a degree cost aren't really a factor in determining what can be built, so they go all out and can get the risk factor down to just about 0.
Can't do that in an aircraft where you're restricted severely by both weight and size and to a large degree cost as well (make it too costly and you no longer have a competitive product), and that's before even considering materials which mean that within the size and weight restrictions you can't get more than a certain strength no matter the cost.

Sadly, as we learned at Fukushima, nuclear reactors are not fortresses, and they can be damaged badly enough outside the armored containment to make them vulnerable to flawed emergency management...  I remember seeing the raw video of a manned recon into the isolation condenser room at Fuku I unit 1... And seeing the "percent full" gages on each tank standing at 65% and 83%... in the confusion, they had shut off the ICs, which are (otherwise) so much of an "I win" button that the next gen of reactors will have them...

"it's for the extremely remote chance that a large asteroid falls onto the dome" -- I doubt that the domes are intended to deal with large asteroids.  The best way to protect a nuke plant from a large asteroid is to build the plant on the other side of the Earth from where the asteroid hits.  But maybe the dome would protect against a small meteorite fragment, the kind that now and then crash through someone's roof or smash someone's car.

The "aircraft crash" scenario is a real design case. The "large asteroid" is almost certainly not.

@Harper the containment vessel however worked as intended. It was the "soft structures" surrounding it that took the damage, which included a cooling pool full of fuel rods as of course the flooding happened at the worst possible moment, just when one reactor had been emptied of its fuel rods but the replacements not yet installed, so there were 2 loads of fuel in that pool.

@WayneConrad large is relative here, obviously.

All of the existing answers are very good, but let me try to answer a more abstract question: why do any accidents happen?  For example

It's a requirement that bridges don't fall down, and civil engineers know how to make bridges that don't fall down, but occasionally bridges do collapse.
It's a requirement that cars be able to withstand crashes, and the car companies do know how to make cars safer, but occasionally people die in car crashes.
It's a requirement that food be safe, and we know how to cook food to kill bacteria, but some people still get food poisoning.

The answer to all of these is basically the same as the answer to your question.  There is an inherent risk in everything.  Risks can be mitigated, but at a cost. The more you want to reduce the risk, the more expensive it becomes. Getting the risk down to absolutely zero would have an essentially infinite cost.  For any given situation, at some point, somebody (either an individual consumer, or a government regulator, or just society in general) has decided that reducing the risk any further is not worth the increased cost. The cost-benefit tradeoff may not have been done consciously, but it has definitely been done.
For example, according to the CDC, about 3000 people die every year in the US from food poisoning. Given that the US population is about 300 million, you have a 1 in 100,000 chance of dying from food poisoning this year.  If I told you that I could reduce your chance of dying from food poisoning to 1 in 1,000,000 but you have to pay \$50 for a hamburger from your favorite fast food joint, instead of \$5, would you do it?  Probably not. The risk is already really low, and you'd rather spend that \$45 on something else.  So you buy the \$5 hamburger and take your chances.
The cost benefit tradeoffs often change over time.  If new technology evolves that allows risks to be reduced for less money, risk goes down.  If the public demands a lower risk and is willing to pay more money for it (e.g. $50 hamburgers), then risk goes down.

The general public also doesn't realize just how much work is done to minimize such risks. To give one example, there is an international committee of industry experts still actively working on how best to prevent a repeat of one air crash (which was caused by a problem with the  material used to manufacture a component), nearly 30 years after the crash occurred. The detection rate from inspecting 100% of similar material  to prevent a recurrence is about one per year - and most of those are false positives. This sort of thing is way outside most people's real-life notion of "risk".

@alephzero United 232?

Having worked in an engine overhaul shop for an airline for years I will add my 2 cents.
First, "cowling" is the wrong nomenclature. A "cowling" applies to the cover or surround of a fuselage mounted engine, although "cowling" is commonly used to refer to any engine cover.
The cowling, or nacelle, is not meant to contain anything, it is for aerodynamics and might contain possibly birds and similar injested debris *not going through the core engine"....... maybe.
It is the "Core engine case" that is designed to contain most departing parts under most conditions. Note I said "most", NOT "all". This is heavy metal like a car's engine "block" but of different metals. Generally, its the compressor/turbine blades that fail, and primarily from ingested debris (and fatigue) and forward broken blades moving aft through the core that takes out more blades. Think about it. The engine moves AIR, and in so doing the AIR is moving through rotating discs of blades and not much else. Its these blades and discs rotating at upwards of 30,000 rpms that can be damaged, come apart explosively and must be essentially contained in the core case in destructive tests.
The forward most blade or fan blade is really like a ducted prop. Should the parts holding it on or the shaft connecting it to a turbine blade/s fails, the fan can/WILL depart the a/c like a buzz saw taking off the entire inlet section forward of the blade and slicing through anything it contacts.
Either failure will occur explosively, scattering many small and large pieces at tremendous force in ALL directions, like a bomb. Nothing man builds can be perfect against the  forces of nature, all the time.
I have seen A/C return with nothing but an empty nacelle where an engine was when the A/C departed. The A/C had holes in it all over the fuselage, wings etc. The entire core engine departed the airframe leaving a hollow nacelle!


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Where do I find the Weight Variants of an Airbus aircraft?
Where do I find the Weight Variants (WV) of an Airbus aircraft?

The weight variants for each model are listed in the aircraft's type certificate which describes the type's compliance with airworthiness regulations. You can look up the type certificates from regulators such as EASA and FAA.
As an example you can look at the document for the A330. Each of the minor models (-300, -200, and -200 freighter) has its own section, and the weights are listed in the "Technical Characteristics and Operational Limitations" subsection. Below is the table showing some of the A330-200 weight variants. The maximum takeoff (MTOW), landing (MLW), and zero-fuel (MZFW) weights are defined for each variant.


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Is a wind speed of 150 km/h dangerous during a flight in an Airbus A321?
I am just curious about this... during one flight on Airbus A321 we experienced turbulence and the pilot told us that the wind speed is 150 km/h, so we won't get drinks and snacks due to safety reasons. The question is: Was that wind speed 'normal' or 'dangerous'? What could happen?

I can assure you there's nothing dangerous about a high wind speed on its own, the 'wind' the aircraft is facing is always the same as it moves relative to the air. Kind of like a boat on a river, if it's going up or downstream, it'll move slower or faster relative to the shore, but it'll be moving with the same speed across the water surface. As for whether it's normal (I think jetstream wind speeds can actually go higher than that), I'll leave that to the meteorologists among us. :)

FWIW, 150 km/h isn't that much wind at altitude. I'm used to thinking of wind in knots. 150 km/h is approximately 80 knots, and when crossing the North Atlantic in MNPS airspace during the winter, which was arranged to take advantage of the winds eastbound, wind speeds in excess of 100 knots were common.

@Terry true, but you won't find many A321s crossing the north atlantic, let alone loaded with passengers, and even fewer in winter :)

@jwenting Actually, A321s do cross the Atlantic in regularly scheduled passenger service via Iceland nowadays. See WOW Air.

@jwenting British Airways even flies their A318s across the Atlantic from London City Airport to JFK via Shannon, Ireland and direct from JFK to London City.

High windspeeds themselves are not a problem, but the environment surrounding them may be.
For example, in the wintertime it is not uncommon to find a strong core of fast winds embedded within the polar jetstream.  We term this windspeed maximum a 'jet streak'.  There are a couple of meteorological phenomena associated around these jetstreaks, such as regions of ascent and descent in the entrance and exit regions of the jetstreak and often times turbulence in the periphery.  You'll also find regions of turbulence on the lateral extent of the jetstream (polar side in particular).  Lastly, if you are flying at an angle to the jetstream or perpendicular to it, you will often experience changing wind direction along with speed, and this can be turbulent.
There is nothing dangerous about that wind speed alone.  What likely happened is that other airplanes were reporting turbulence and the pilots were being cautious with you and just offered that wind speed up as an easily understood number to justify the precaution.

It's not unusual for aircraft to experience a lot of wind at altitude. In fact, there's a website dedicated to speed records- all of those shown would be impossible without the help of the wind boosting the aircraft in addition to its normal speed. Many people have unknowingly travelled at a velocity faster than the speed of sound on the ground when crossing the Atlantic.
While I'm not too familiar with it, I think that jetstreams and the like have a tendency to develop more air pockets and turbulence than calm air, including clear air turbulence, where there are no visual cues before the turbulence hits.
Many incidents happen when people are unprepared for a bounce at cruise altitude, probably hence the pilots decision. A broken ankle is not unheard of. As for catastrophic consequences, they are very rare. Typically, the pilot will slow the aircraft down a little to reduce shaking and stress on the airframe.
Here's an incident on a Cathay Pacific 747 around a month ago, midflight:

Passengers reported the turbulence appeared to have lasted for about 2
  minutes, at the time the fasten seat belt signs were not illuminated,
  passengers and cabin crew were moving through the cabin, when the
  aircraft suddenly and unexpectedly jolted throwing passengers and
  cabin crew to the ceiling causing damage to ceiling panels and
  overhead lockers. Aviation Herald

Well, not supersonic :) but a ground speed higher than the speed of sound

Haha, yeah, I remember flying on a BA 767 from KSFO to EGLL with a G/S of 1030 km/hr with a 160 kmp/h tailwind! But there wasn't any indication of turbulence and the seatbelt signs stayed off.

The reduction of speed is not to reduce the shaking and won't have much effect on it. It is to avoid exceeding the maximum permitted speed (Vne) in gusts of head wind.

That Cathay Pacific incident is an excellent reminder of why you should keep your butt in your seat with your belt fastened during the flight unless you've got a good reason to be up and moving around, regardless of what the signs say...

"pockets of turbulence" is a misnomer. turbulence is caused by wind shear (or gradient), which is defined as the difference in wind speed and direction over a relatively short distance in the atmosphere.

To put wind speed in perspective (further to the previous remark that 150 km/h is not fast at altitude): The speed of the Jet Stream is typically 100 kts (185.2 km/h) but can reach 200 kts (370.4 km/h) over North America and Europe in the winter. Speeds of 300 kts (555.6 km/h) are not unheard of, particularly over south-east Asia. Aircraft flying west-east will try to exploit the Jet Stream to increase ground speed and, similarly, aircraft flying East-West will plan to avoid the Jet Stream.


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Why would a canard wing be more prone to icing than a stabilizer?
Someone said a canard wing is more prone to icing than a stabilizer. If true, what would explain this?

"someone" who? when? where? link?

"a friend" :-) an answer stating it's not correct or determined by other factors would be just as accepted

It has to do with the local speed around the airfoil.
In a canard configuration, the forward wing has to produce more lift per area than the main wing in order to have natural longitudinal (or pitch) stability. This means the suction is more intense, and the pressure on the upper side of the airfoil right behind the leading edge is lower than on the main wing. Also, the angle of attack of the wing is higher, presenting a larger area to the oncoming flow. Large droplets of supercooled water will simply splash on the leading edge and freeze there instantly, causing clear icing.
In a conventional aircraft the tail surface will provide very little lift, and in many cases even produces a small downforce. Now the main wing shows lower pressure on the forward part of the airfoil, just where ice collects the most. In the special case where the tailplane is in the wake of the wing, some of the water droplets will have frozen to the wing already and less water will be available to cause tailplane icing.
The capacity of air to hold water depends not only on temperature, but also on pressure: The lower the pressure, the more likely it is for condensation to happen. Condensed, super-cooled water, or liquid water hitting a cool surface, will form ice. Now you have both components needed for the explanation.
Pitch stability is the explanation for the higher propensity of canard wings to collect ice than conventional tailplanes!

If the canard has a thinner leading edge compared to the stabilizer, this could happen. From BEA Systems Think Ice:

A relatively large radius aerofoil at moderate or low airspeed creates a larger pressure wave ahead of the leading edge, which forces the air around it,
  carrying most of the moisture with it. Only droplets sufficiently heavy to overcome this flow will impact on the leading edge. Thus, a large chord aerofoil with a blunt leading edge has low ice accretion efficiency.  Conversely, a narrow radius leading edge generates a smaller pressure wave and so the accretion rate is greater.

Note that by the same token, the stabilizer is more prone to icing compared to the main wing.

Of course the leading edge radius has also a role in icing characteristics, because a small radius will cause higher suction peaks (= lower pressure) on the top surface than a large radius. But it can not explain the relative difference in icing between canard wings and tailplanes.

@PeterKämpf smaller radius also causes larger water vapor gradients (than around less curved surfaces) and will be preferential locations for ice growth independent of the pressure anomaly.

@PeterKämpf You're wrong, it can indeed very well explain the difference. In aircraft icing the total accretion is heavily depended (not only in volume but in the form of the ice shape etc.) on collection efficiency. You speak about suctions peaks, pres. grads etc. but you're forgetting that we talk about multi-phase flows. To put it simple, if you have no water there, it wont freeze. The combination of airfoil thickness and the distribution of droplet's mean volume diameter in the cloud are major factors in aircraft icing. Also, for conventional config. the wing is "shielding" the tailplane.

A relevant paper -> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050186904.pdf


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How often is the fuel sampled in turbine aircraft?
In planes using avgas, sampling fuel from the tanks is generally done before every flight, or at least before the first flight of the day and then after any refueling. This is done because water contamination is fairly high risk. Gasoline does not mix with water, so it collects at the bottom of the tank, and because it does not burn, when it gets to the engine, the engine will quit.
Water does mix with kerosene though so this isn't as big risk and it can't be checked this way anyway because it won't collect in the sump. It could detect other contamination though. For example in this incident hugely excessive addition of biocide caused damage to the engines, and in this case the report says the biocide did form a separate layer below the fuel.
So how often is fuel in turbine (turboprop and jet) aircraft usually sampled? And is it subjected to any additional tests beyond visual inspection?

Water in fuel accident: British Airways B772 at London on Jan 17th 2008 http://avherald.com/h?article=4270d893

@DDuck not really. That accident was caused by problems in the FOHE (Fuel Oil Heat Exchanger) design causing it to ice up under certain conditions. The fuel was not contaminated with water: "Fuel samples were analysed and showed [...] a natural allowable water content between 35 and 40 parts per million. There was no contamination of the fuel." (from the site you linked)

Water and microbiological contamination are a threat to turbine aircraft even if water mixes with kerosene based fuels:

Industry-wide, the major threat to FQIS [Fuel Quantity Indication System]
availability is microbiological contamination of
the fuel (see FAST 38) and water in the fuel tanks.
Settled water in the fuel tanks may affect FQIS
indications, typically driving FQIS probe readings
out of limits and leading to aircraft delays for tank
draining. Therefore, a regular water drain task
is essential for smooth aircraft operations.

(Airbus FAST 42)
The reason is that both water and microbiological contamination in the fuel lead to a higher dielectric capacitance, which is used for fuel quantity sensing:

The FQIS system is calibrated for
use with jet fuel and the probes
measure from 'unusable' to full
tank capacity. However, the
dielectric of water is approximately eight times higher than the
jet fuel dielectric. Therefore, when
there is water in the proximity of
the probes, the fuel measured capacitance changes. Typically, the
probes will read a higher capacitance level than the actual fuel
level.

(Airbus FAST 42)
If only a small amount of water is suspended in the jet fuel, it can be safely fed to the engines without causing a flameout. However, water collecting at the bottom of the tanks must be drained:

All modern civil
aircraft incorporate water drainage
and/or scavenge systems to help
achieve this. The Airbus wing
design ensures water is maintained
in suspension and fed to the
engines; any remaining water is
then drained at regular intervals
via the water drain valves.

(Airbus FAST 38)
For an Airbus A320, this fuel tank draining must be performed at least once every 36 hours:

The amount of water drained is about 200 ml per week and per tank on an A340 after improvements done in 2007 (Airbus FAST 42).
Airbus recommends to visually inspect the drained water and to perform a fuel analysis at least once a year to detect contamination:

A second key element of prevention is monitoring. Often overlooked is the need to visually
inspect any water resulting from
the MPD water drain task. Drained
water and fuel will separate within
the container with the water
settling to the bottom. Both should
be clear with no particulates
evident to the naked eye. Any
'cloudiness' in either fluid indicates
possible contamination and action
is required.
Currently, Airbus recommends an
annual analysis of fuel from each
aircraft to test for fuel/water contamination, which should be considered a minimum for all operators.

(Airbus FAST 38)
Should the visual inspection show any contamination, or should the pilots notice discrepancies in their fuel analysis (see e.g. here for example calculations), or should the FQIS show fluctuations or degrading (fuel quantity indicates XXX), a detailed analysis should be performed according to this flow chart:

(Airbus FAST 38)

My understanding of the last quote is that the drained water should be visually inspected every time, and the once a year thing is the more thorough analysis to determine the level of biological contamination.

@JanHudec You're right, it sounds like the "action is required" would then be the full analysis of the fuel. "Often overlooked" sounds like this visual inspection isn't done every time, but they recommend doing it.

Indeed. This suggests in the incident I referenced they should have done the draining at least once (it was two days between the biocide treatment was completed and the incident flight), yet somehow they didn't notice or didn't follow up on the unusual brown color (mentioned forming in the samples taken after the incident) of the drain.

Flying single and twin engine turboprops, it is our practice to check sumps and drains in an identical manner as with gasoline fueled aircraft. Typically this is done at each leg that the engines are shutdown.
The manner for inspection is identical.  Generally, fuel is collected, and not dumped on the ramp due to the less volatile fuel not evaporating as quickly (although collection of gasoline sump draining is implemented at many airports).
Operations are conducted under Part 91, and this comment is focused on general aviation fleet operations.


text:
Is it possible to access the baggage compartment in flight?
In this incident the cabin crew entered baggage compartment to find and secure a smoking suitcase. But that was in ATR-72 which has the baggage compartments on the main deck behind cockpit and aft of cabin and passage between cockpit and cabin goes through the (larger) forward compartment.
In this incident it was clearly an advantage to be able to get to the cargo. Would it also be possible in some larger planes? Is there any regulation or guideline explaining why it should or shouldn't be possible?

The article says the smoke was from acetone and a hairdryer but "The female owner of the suitcase reported it was a fluid to make the hair lighter." That sounds much more like hydrogen peroxide (hair bleach) than acetone (nail varnish remover).

There are 4 classes of cargo compartments listed in 14 CFR 25.857 for transport category airplanes. The main concern with in-flight access is in regards to fire safety.

Class A: Crew will easily discover fire from their station (and therefore be able to suppress it as well)
Class B: There is a fire/smoke detection system, and access for crewmember to use a hand fire extinguisher
Class C: There is a fire/smoke detection system to alert the crew, and a fire extinguishing/suppressing system installed
Class E: For cargo-only aircraft (no passengers): There is a fire/smoke detector, and a way to shut off air flow to the compartment

So basically, there has to be some way to both detect and address a cargo fire. Class A compartments are those such as in the cockpit where a detection system is not necessary. It sounds like the ATR example would be Class B, since they had a fire indication, and the crew had access to extinguish the fire themselves.
In larger planes with cargo in the lower hold, the compartments are usually Class C, so they have a detection system and a suppression system, and in-flight access is not required. As David Richerby's answer notes, the compartment may be accessible, but that doesn't mean that crew could effectively locate or fight a fire in those areas. Especially for large cargo holds, a fire suppression system installed througout the hold is much more effective than crew members gaining access at certain locations. This also provides the benefit of isolating the main cabin from the fire extinguishing agents.
Besides fires, there are even rarer instances where cargo compartment access could be helpful. But even in that case, it was better for the plane to just come back and land to access the cargo compartment.

No elevator to the cargo deck like in thriller movies? I'm disappointed :( /sarcasm

In general: Class C's are underbelly holds on jets, which may or may not be accessible through say an E&E bay or a hatch from above.  Class A's and B's MUST be accessible for fire extinguishing -- Class A is used for cockpit storage compartments, and Class B for the small main deck cargo compartments on regional and commuter turboprops (like the ATR-72 mentioned in the question).  Class E cargo compartments are the main decks of cargo aircraft -- although the requirements for them are a-changing in the wake of UPS 6.

@tohecz the L1011 had an elevator!

I remember in the Arnold Schwarznegger movie 'Commando', he does into the cargo hold, from the back of the aircraft and escaped through the landing gear

Is Class D skipped? Or was there a legacy Class D?

@curious_cat there is no "Class D" but it does leave part (d) there as "Reserved", possibly to allow for future use.

Is the last link (latimes) dead? Wouldn't load for me.

I've no idea how accurate it is but this Straight Dope thread contains confident assertions (some from people who claimed to have worked on these planes) of the following:
Cargo hold inaccessible. BAe146, DC-9-30.
Forward cargo holds accessible. 737, 747.
All cargo holds accessible. 777.
Note that, on larger planes, a lot of the luggage will presumably be in containers so getting into the cargo hold isn't necessarily useful.
This was originally posted as a comment in the hope that somebody would post something more authoritative as an answer.

I can confirm that the forward cargo hold on 747-100/200 aircraft is accessible in flight. To get there you pull up a small section of carpet on the forward part of the maindeck, open the hatch underneath it, and descend into the electronics bay. If you then go forward, you'll be in the compartment in which you can manually lower the nose gear. If you go aft you'll go into the forward cargo hold. There's a heavy fabric wall between the electronics bay and the forward hold that has provision for getting through it.


text:
How often do runways need to have typical maintenance?
At a large airport (think Heathrow, JFK, O'Hare), I would imagine that the runways take a lot of abuse from all the huge planes landing on them.  So, a quick multipart question:

How do large passenger planes damage runways?  Does it differ depending on the surface?
How long can a runway sustain this damage before the runway must be repaired or resurfaced?

Keep in mind, I'm talking about normal wear and tear.  So, I'm not really wondering how long it takes to repair a runway after a 767 lands with it's gear up, for example.  Rather just wondering how the normal maintenance cycle works.

Not sure what the maintenance programs are at the major airline airports, but I know the runways and taxiways at KFRG get an annual "major maintenance" (joint sealing, painting if they need it) - they're usually closed for the better part of a day while the crew runs around like a bunch of ants on crack making sure the surface is in acceptable condition. I imagine at some point they'll need a full resurfacing, but good ongoing maintenance probably lets them defer that.

related: http://aviation.stackexchange.com/questions/2375/do-landing-strips-suffer-from-potholes-or-rutting

The pavement requires frequent renewal of the top surface. The frequency depends on the type of pavement and on the weather. Asphalt for instance suffers a lot of damage when it snows and becomes less viscous when it is very hot, causing heavy aircraft to sink.
The main issue with repairing a runway on a major airport like JFK or O'Hare is doing so with little to no impact on traffic. Resurfacing an entire runway can take a long time. Therefore, rather than closing the runway for days at a time, airports try to do as many short maintenance operations as possible at night.
Some operations cannot be done in a single night but can be divided into several perfectly timed tasks, that can be done between 11 PM and 5 AM. Some operations can not be divided into such small tasks and require that the airport closes for a day or more.
The French airport Orly (ORY) for instance has one slab of concrete replaced every night on the runway. This way, the strip can be renewed every other year on average, without disturbing traffic. (I saw a video on this subject once, but can't seem to find it again.)
The results of each runway inspection are compared with the results of the previous runway inspection. Maintenance programs are revised based on the rate of deterioration of the runway. The FAA has various resources to help airport operators plan these maintenance operations.
But damage done by huge planes is only one of the many things that needs to be taken care of on a regular basis. Moreover, aircraft manufacturers try to design landing gear that reduces the impact of the aircraft on the runway as much as possible (note that airports can charge higher landing fees for aircraft that do more damage to the runway, so airlines want airplanes that don't damage the runway).
The most common maintenance operations are (not in any order, and indicated frequencies are only a rule of thumb, as these will change from one airport to another):

Removing rubber. This is an important one. When taking off or landing, aircraft often skid for a brief moment and leave rubber on the runway. This greatly reduces the adherence of the runway, which has a negative impact on stopping distances. This is usually done with high pressure water.
Cleaning the approach light systems and other visual aids. These get dirty with rubber, tartar, dirt... This operation can be done for instance with a mix of water and sodium bicarbonate, or by using dry ice.
Checking the quality of the approach light systems and other visual aids. Often, a truck with optometric sensors drives down the runway and measures the luminosity. Sometimes the lights need to be replaced. Airports usually have spares available for every type of light that is used on the runway.
Paint and markings somtimes have to be redone.
Evaluating the pavement and surveying the structural condition of the runway. Trucks are equipped with all kinds of sensors and go up and down the runway. The measures are used to make a map of the condition of the runway. Sometimes, the runway is divided into sub-sections and a different section will be evaluated every month or every other month, so that each section is inspected twice a year.
Checking for foreign objects on the runway. On all the airports I visited, this is done by the airport firetruck several times a day. Every few months, they sweep the runway to prevent grass, dirt, pebbles,... from accumulating.
Repairing cracks and resealing joints on the runway. This is to prevent water infiltrations that have long-term catastrophic consequences. A visual inspection of the runway is performed daily to look for cracks.
Cutting the grass around the runway and on the airfield. The grass has to be kept 2 to 4 inches high around the runway. This reduces the amount of debris that can be blown onto the runway. This also reduces the number of birds and other animals living close to the runway (because nobody wants birds to be ingested by an engine, or a squirrel to be run over).
Keeping all water evacuation paths clear. No water can be left to build up on the runway, since this would increase stopping distance for aircraft, and may freeze.

There also are seasonal operations, such as de-icing the runway or removing snow.

Rubber residues are mainly created when the non-turning wheels touch down on the runway and skid for short while until the friction overcomes their moment of inertia and they reach appropriate rotation speed.

@usernumber Ask and ye shall recieve: http://aviation.stackexchange.com/questions/9436/what-do-they-use-to-de-ice-aircraft-and-runways-on-snowy-days

@usernumber
Doing research in 2023 for ENW, your post was the top hit. I did find a video posted in 2021 of ORY's runway replacement process. I hope it's the same or similar to the one you reference. https://youtu.be/gPjJd99UP9A Even tho 8 yrs old, your information is still helpful. Packerbackercat7


text:
How heavy is too heavy to land on a grass strip?
This is actually a tricky question, because it's not just about gross weight, but about how much weight each individual tire imparts into the ground.  This, of course, depends on the design of the suspension and the number of wheels.  Thus a really big craft, with a well designed undercarriage, can land on grass just fine (like a C-130 for example).
The point being, "how heavy is too heavy" has more to do with specific weight supported by each wheel during landing, and less to do with total landing weight.
So, kind of a two parter.  Firstly, how much weight can a grass field generally support when an aircraft is landing?  Secondly, how do aircraft designers make landing gear that can land on a grass field without plowing into the ground and ripping off?

Some related info can be found in this answer.

The key parameter to watch is tire pressure, not weight. Everything below 80 psi should have no problem on a dry grass strip. Gliders have around 50 psi and use grass strips (and the next available field, if needed) routinely. When tire pressure exceeds 100 psi, I would look more closely at the grass strip - if the soil is hard enough, why not land there?

Your second question is really easy to answer:
Q: How do aircraft designers make landing gear that can land on a grass field without plowing into the ground and ripping off?
A: Big Fat Low-Pressure Tires.

A wide, low-pressure tire has a larger contact patch with the ground, spreading the aircraft's weight over that larger area helps prevent the tire from digging into the ground, and lets the aircraft roll more easily on rough or soft surfaces.

Your first question - How much weight can a grass strip support? is a much tougher one.  The bearing strength of an unimproved surface depends on a number of factors (some big ones: The composition of the runway bed, its drainage, and what kind of surface cover - turf, rocks, etc. - it has).
If you look at aerial photos of Edwards Air Force Base you'll notice part of the runway complex includes a dry lake bed:

Some pretty heavy stuff has taken off from and landed on those runways - space shuttles have landed there.
Similarly TACA Airlines Flight 110, a Boeing 737, landed on a levee at NASA's Michoud Assembly Facility (and was subsequently repaired and took off again, though they used an access road for the takeoff rather than the grass levee).
Conversely if you tried to land a Cub on a grass strip that's saturated with water after a rainy month there's a chance your tires might sink into the mud.
CBR Technologies has a nice presentation on unpaved runways, which includes some discussion of how their bearing strength (and thus maximum landing weights, tire pressure limits, etc.) can be calculated, but each unimproved runway will be different, and the values will likely change over time.

Excellent answer.

If you have 24 wheels to spread the load out, even 400 tons might not be too heavy. Especially when the tire pressure can be adjusted from the cockpit (however, reducing tire pressure quickly lowers the MTOW of the An-124). See here for some hearsay on the issue.
The aircraft designers keep tire pressure low to enable this. Everything below 80 psi should have no problem on a dry grass strip. Gliders have around 50 psi and use grass strips (and the next available field, if needed) routinely. When tire pressure exceeds 100 psi, I would look more closely at the grass strip - if the soil is hard enough, why not land there? The C-130 has a tire pressure of 125 psi (8.6 bar) and routinely operates from unpaved strips.

Polish Air Force C-130E with serial 1502 during grass strip take-off and landings on 4 April 2014
(Image © Capt. Włodzimierz Baran / Dowódca Generalny Rodzajów Sił Zbrojnych)
Here is a video of an Illyushin 62 landing in a field. Note that it was not anywhere near its maximum flight mass at this event. It was the last flight of this bird; now it sits idle as a tourist attraction in the German countryside.
However, I once landed an ASW-20 C on a muddy field soaked after days of rainfall. The wheel immediately dug in and the aircraft slid for 20 meters along the bottom of the forward fuselage. It took me a week to repair the cracks - the ASW-20 has a great landing gear, but is poorly suited to plough like this through a muddy field.

The operative term to look up here is ground pressure.  This is simply the force required to support the object divided by the area that force is spread over.  You can reduce the ground pressure of an aircraft by increasing the size of the landing gear - most obviously, using more wheels and/or larger wheels.  On low-friction terrain (ice, snow), try skis - they're lighter.
The ground pressure of a large Main Battle Tank at rest is about 15psi.  This is very low, even though the tank is very heavy, because its weight is spread over a pair of very long and broad caterpillar tracks.  This allows the tank to move and fight effectively on almost any terrain short of an actual river.  The ground pressure of a normal car is more than twice that, and is closely related to the tyre pressure.  Some off-road vehicles have very large tyres that can be partially deflated for this reason.
The actual force to be supported by the ground is not just the weight of the aircraft, but the dynamic forces caused by descending onto the ground during landing.  These are obviously reduced if you make a very smooth landing.  Dedicated STOL aircraft have many features to control their trajectory at low speed to assist with this, such as blown flaps and even tilted engines.  Their slow forward speed at touchdown also minimises the effects of the wheels digging into soft ground.

Any aircraft can land on a grass field. It is actually easier and safer to land on a grass field than a hard surface. The reason that hard surface runways are used is because a dirt runway will develop ruts and require frequent resurfacing, but a hard surface can be landed on repeatedly with no change. Also, if you land on dirt it kicks up a lot of dust and possibly rocks which are bad.
Landing gear will not "rip off" an aircraft unless it hits an immovable object, like a brick wall. The landing gear is the toughest part of a plane, so if you hit a brick wall what happens is that the plane rips off the landing gear, not the other way around. I know of small planes that have hit steel highway dividers and come to a dead stop. You have to have a very severe sort of accident to pull apart an aircraft from the landing gear. Normally wings and stuff will come off way before the landing gear does. The landing gear is heavily integrated with the fuselage, so they basically act like a unit.
Dirt is very good for landing because the aircraft comes to a stop more quickly. With concrete you just keep rolling and rolling, which is much more dangerous. If I had to land off field, I would look for the muddiest place I could find. Shallow water is excellent too for the same reason: it slows you down quickly.

To respond to some of the asphalt-loving comments... In any off field landing you should ALWAYS land on dirt, grass or mud (even better). Landing on streets and highways is incredibly dangerous because there are cars and people on roads and other dangerous obstacles such as railings, signposts, fire hydrants, traffic lights and highway lights. Also, in any settled areas there will be wires and steel cables strung across the roads which are invisible from the air and can cut a plane in half.
People who advise landing off field on asphalt are just power pilots who don't have off field experience, so they don't know what they are talking about. As any glider pilot or bush pilot will tell you grass is way way safer than trying to land on a road. Even at an airport, it is much safer and easier to land on the grass strip at the airport than the runway.

This is terrible advice, to the point of being downright dangerous. If the aircraft digs into the soft surface then it will stop much more abruptly, which is potentially very dangerous, and there's the danger of flipping or losing directional control on the ground. Standard procedure for a gear-up landing - for example - is to find the longest, widest paved (or at least smoothest) runway you can; it's the standard procedure for a good reason.

Do you have any evidence to support this?

There is an AD on Beech 35, 36, 55, and 58s due, in part, to stress incurred from unimproved strips (particularly in the 55/58).  I would say that soft fields are safe, but not safer than a nice wide paved runway.  As far as the rolling problem goes, Cleveland brakes do work well.  Changing pucks is easy and they're cheaper than spar doublers.


text:
Why fuel left/right when C172 is secured?
The C172 POH checklists say to set the fuel selector to "right" or "left" when securing the plane but "both" while operating the plane.
What is the point of changing the fuel selector when the engine isn't running? IOW, what practical (not legal) problems could it cause to leave it on "both" while secured?
[Edited to remove incorrect understanding of the vent connection between tanks, which is cleverly designed to not cross-feed fuel.]

Although it's a slightly different question, the top answer to Cessna 172/182: Should I set the tank selector to the R.H. tank after parking the airplane? explains why

It's pretty much to limit imbalance when parked on a slope, and if the tanks are full, stop fuel from the high side tank from forcing fuel out of the low side tank's vent or seeping out the fuel cap if it isn't completely fluid tight.
On the '68 Cardinal I used to own, the vents went straight to the wing tip on each tank's side.  If parked on a side slope with fuel on both and full tanks, you might end up with a third of the high side's tank volume on the ground under the left wing tip (later they were cross vented to prevent this).

This diagram from the POH shows the tanks are cross-fed separately from the selector, so it seems like that wouldn't solve the imbalance issue?

It doesn't show the internal valving.  If the selector is set to Left say, the left tank is connected to the engine and the right tank is completely isolated, so yes it will.

The line at the top (directly between the tanks, not going through the fuel selector) has valves on it? How do they work?

It just connects the tops of the two tanks together.  Allows the right tank to vent to the left tank but they actually connect to each tank on the outboard top of each so fuel can't migrate through.  The venting fuel caps are used as additional protection against a wasp or whatever blocking the main vent.

Ah, it all makes sense now. Thanks!

On most C172s, the fuel selector cross-feeds in the BOTH and the OFF position.  The LEFT or RIGHT position do not crossfeed, which eliminates cross flow of fuel, except when the tanks are nearly full.  Most older C172s can do a small amount of cross feed between the tanks through a vent line connecting the tanks. The external vent is near the strut on the left wing. If the plane is parked nearly full, with the right wing high, the vent will discharge fuel as the fuel level rises in the left tank.
So a topped L tank can transfer some fuel to the R tank, and reduce the fuel in the L tank.  Something to watch out for when fueling and managing fuel.
Probably a safer parking configuration would be to park with the fuel OFF, but unfortunately, on most C172s this means that the tanks can cross-feed.  Therefore, the LEFT or RIGHT setting are preferred.

Thanks for pointing out the fuel selector can cross-feed in the "off" position; that's an unexpected (mis)feature.